1. Field of the Invention
The present invention relates to a half-tone image generator, and particularly to a generator capable of low-cost, rapid generation of a pseudo-representation of the tones of a multitone image as sets of black and white bitonal data.
2. Description of the Prior Art
The term "half-tone image generator" as used herein refers to apparatus that provides a pseudo-representation of the tones of multitone images as sets of black and white bitonal signal levels. Displaying black and white bitonal images simply by decreasing the number of tones results in the appearance of spurious contours, i.e., contours that are not present in the original image, producing a major degradation in the quality of the image. Dithering is a technique of de-emphasizing such spurious contours through the addition of noise.
Various dithering methods have been proposed corresponding to the number of tone levels or colors in a processed image. Ordered dithering and error diffusion are two of the principal methods used.
In ordered dithering, noise is added to multitone image data in the form of periodic signals. Error diffusion consists of using a given value to binarize multitone image data and utilizing a feedback of the difference between the original signal and the binarized signal to minimize error.
However, as ordered dithering uses the addition of noise in the form of a signal having periodicity, when a document printed using halftoning is scanned and utillized the ordered dithering, interference between the halftone pattern and the periodicity of the ordered dithering gives rise to a moire effect, a pattern of regular stripes that does not exist in the original document.
To a certain extent, error diffusion can depict shade tones naturally without the striped effect associated with ordered dithering. However, error diffusion involves real-time two-dimensional image processing, which requires complex, costly apparatus and increases the processing time.